1. Field of Invention
This invention is directed to techniques that use position-sensitive detectors, such as in measuring wavelength of light, in measuring wavelength shift of light, or in producing a chip-size wavelength detector.
2. Description of Related Art
Spectrometers and wavelength shift detectors are widely used in scientific, academic and industrial applications to measure the various wavelengths of multi-colored light. Known spectrometers use a wavelength dispersive means, such as a prism or grating, to separate components of the multi-colored light according to the wavelengths of the components. The intensity of the wavelength components is then measured by a photodetector.
Wavelength shift detectors may also use wavelength dispersive means. However, instead of measuring intensity as a function of wavelength, they generally measure a shift in wavelength of a single frequency source away from a nominal value. Often spectrometers are used to measure the wavelength shift, however the wavelength resolution is typically poor, on the order 10−2 nm. For applications where high wavelength resolution is needed, a Mach-Zehnder type wavelength meter is sometimes used.
FIG. 1 shows an exemplary view of a known embodiment of a spectrometer apparatus disposed to measure the intensity of light 11 produced by a source 10. Source 10 outputs a beam of multi-colored light which is imaged by an imaging lens 20 onto entrance slits 30 of a spectrometer 40. The slits 30 are used to define the input trajectory of the light upon the optical elements of the spectrometer 40. The intensity of the light is measured by a photosensitive detector 50, for example, at an exit aperture of the spectrometer 40.
FIG. 2 shows the internal and external components of the spectrometer apparatus of FIG. 1 in further detail. The slits 30 admit the beam of light 11 containing at least two wavelength components, λ1 and λ2. For clarity, only the central ray of the beam of light 11 and no optical components within the spectrometer are shown in FIG. 2. The beam of light 11 is incident upon a wavelength dispersive means 35, shown in FIG. 2 as a grating, which reflects the light with a different phase angle, according to the portion of the grating that reflects the light. The light having a wavelength and phase angle such that the wavefronts constructively interfere at the detector, are measured by the detector 50 as having a given intensity. Therefore, the detector 50 may record the intensity as a function of wavelength, thereby producing the spectrum of the source 10.
According to FIG. 2, the photosensitive detector 50 may have a plurality of discrete photosensitive elements, such as a detector array or charge-coupled device (CCD) array. Each photosensitive element may be independently accessible by a data collection apparatus, such as a computer.
While FIG. 2 shows the wavelength dispersive means as being a reflective grating, one skilled in the art will understand that other wavelength dispersive means may also be used in the spectrometer to separate the components of light according to wavelength. For example, a prism will refract the various components at a different angle depending on the wavelengths of the components. However, regardless of the means used to disperse the wavelength components of the multi-colored light, it will be appreciated that all spectrometers must have some significant distance between the wavelength dispersive means 35 and the optical detector 50, in order to give the components a measurable separation.
Another device which can separate wavelength components from a multi-colored light is a Fabry-Perot etalon. An exemplary Fabry-Perot etalon is shown in FIG. 3. The Fabry-Perot etalon 70 is composed of two reflective films 72 and 76, which are applied to the front and rear surfaces of a transmissive cavity 74. Light incident on the Fabry-Perot etalon 70 will be transmitted if its wavelength is such that an integer number of half-wavelengths can be fit inside the thickness d of the transmissive cavity 74. In this case, light having this wavelength will be transmitted, whereas light of other wavelengths will be reflected. The reflectivity spectrum of the Fabry-Perot etalon 70 is shown in FIG. 4. A dip 78 in the reflectivity spectrum occurs at the transmission wavelength of the Fabry-Perot etalon 70. Fabry-Perot etalons are not, in general, used in spectrometers or wavelength shift detectors, because they are not tunable, and thus cannot produce a spectrum, or measure different wavelengths.
The requirement for a distance between the wavelength dispersive means 35 and the optical detector 50 requires a spectrometer to be a relatively large, bulky device. The distance then also requires the spectrometer to undergo frequent alignment and calibration to adjust the angle of the grating or prism relative to the slits and detector, in order to optimize performance. Furthermore, the distance between the optical elements and the detector in a conventional spectrometer makes the spectrometer sensitive to vibrations, so that it must be used in a stable, well-controlled environment.
One approach to making a spectrometer which does not need to be aligned, and is compact and robust, is disclosed in U.S. Pat. No. 5,166,755 to Gat. Gat describes a spectrum resolving sensor containing an opto-electronic monolithic array of photosensitive elements, and a linearly variable optical filter that is permanently aligned with the array. The linear variable filter is a substrate covered with variable thickness coatings formed into a wedge shape.